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Development of a vaporization system for direct determination of chlorine in petroleum coke by ICP-MS
Microchemical Journal 109 (2013) 117–121
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Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Development of a vaporization system for direct determination of chlorine in petroleum coke by ICP-MS Fabiane Goldschmidt Antes a, Eduardo Dullius b, Adilson Ben da Costa b, Rolf Fredi Molz b, José Neri Gottfried Paniz a, Erico Marlon Moraes Flores a, Valderi L. Dressler a,⁎ a b
Departamento de Química, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil Programa de Pós-Graduação em Sistemas e Processos Industriais, Universidade de Santa Cruz do Sul, 96815-900, Santa Cruz do Sul, RS, Brazil
a r t i c l e
i n f o
Article history: Received 30 November 2011 Received in revised form 19 March 2012 Accepted 26 March 2012 Available online 30 March 2012 Keywords: Electrothermal vaporization ICP-MS Chlorine Petroleum coke
a b s t r a c t An electrothermal vaporization system (ETV) was developed for direct chlorine (Cl) determination in petroleum coke by inductively coupled plasma mass spectrometry (ICP-MS). The ETV system consists of a high power lamp enclosed in glass chamber equipped with solenoid valves. The system is controlled by a printed circuit board and software developed in LabVIEW environment. The temperature and heating time were optimized. Solenoid valves were used to control the flow of Ar (carrier gas) during pyrolysis and vaporization steps. Pyrolysis and vaporization temperatures were set at 350 °C and 900 °C, respectively; while the Ar flow rate was 1.20 l min − 1 and the plasma power 1300 W. Up to 5 mg of petroleum coke sample can be analyzed. Calibration was carried out by standard addition whereas a certified reference material of coal (SARM 19) was used for that purpose. The relative standard deviation was lower than 9% (n = 4) and the limit of quantification (10s) of Cl was 3.5 μg g− 1. Results obtained for chlorine in petroleum coke sample were in agreement with those obtained using other validated methods for Cl determination. The time of analysis (approximately 2 min per sample), risk of contamination, analyte losses and waste generation are drastically reduced in comparison with techniques that require sample digestion for chlorine determination. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Petroleum coke is a product of petroleum refining process and the chlorine (Cl) content depends on the petroleum origin or source. Petroleum coke is widespread used as fuel, for electrodes manufacturing, production of chemicals, aluminum and steel alloys. The purity of petroleum coke must be evaluated and monitored for its use in these materials [1,2]. Very few works have been published regarding to determination of Cl in petroleum and its refining products, probably because determination of trace amounts of Cl is challenging owing to matrix complexity and Cl losses by volatilization in the sample preparation step. Acid digestion, usually employed for sample decomposition, is not recommended because volatile species of Cl can be produced and then lost. Therefore, most of the published works about Cl determination in petroleum and refining products deal with the employment of combustion methods in closed systems. Decomposition using Schöniger combustion flask [3], combustion bomb [4,5] or microwave induced combustion (MIC) [6,7] has been employed. Pyrohydrolysis was also proposed as an alternative sample decomposition method for further halogens determination [8–10], including Cl. By using
⁎ Corresponding author. Tel./fax: + 55 55 3220 9445. E-mail address: [email protected] (V.L. Dressler). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2012.03.027
combustion or pyrohydrolysis for sample decomposition, Cl determination can be carried out by techniques such as ion chromatography (IC), inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled plasma optical emission spectrometry (ICP OES). Despite the relatively good results obtained by using the sample preparation methods above cited, they have disadvantages such as high risk of analyte losses, sample contamination, higher cost and relatively low sample throughput. On the other hand, direct solid analysis would be ideal for Cl determination. In fact, direct solid analysis has been employed for Cl determination using inductively coupled plasma mass spectrometry in conjunction with laser ablation (LA) [11–14] and electrothermal vaporization (ETV) [15–18]. These sample introduction accessories have been used for Cl determination in coal by ICP-MS [19–22]. One of the main features of LA-ICP-MS is the possibility to obtain information about spatial distribution of major and trace elements in a sample [23]. However, LA is still not widely used in routine analysis. In the case of ETV, a small amount of solid sample can be placed in the vaporizer, usually a graphite furnace that is heated in order to vaporize the analyte [15]. The resulting sample vapor is transported to the ICP-MS by a carrier gas through a transfer tube. By using ETV as sample introduction system in ICP-MS, the sensitivity is improved and problems related to polyatomic interferences reduced; most part of the water and sample matrix that produced polyatomic ions are eliminated in the ETV before the vaporization of the
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analyte. It was suggested that the use of dry plasma conditions would be better than wet plasma conditions [24]. In fact, interferences caused by polyatomic ions can be strongly minimized using ETV because matrix components are removed during the heating step prior to analyte vaporization. In the case of Cl, this aspect is of great importance because the most abundant isotope of this element ( 35Cl) is strongly affected by interferences caused by 34S1H+ and 16O18O1H+ [10]. Different types of ETV have been produced, including a tungsten coil where Yan et al. [25] determined Cl, Br and I, being the ETV system coupled to an ICP-MS instrument. In this study, sample solutions were deposited on the filament coil, dried, vaporized and then the vapor transported to the plasma. The authors observed that Cl, Br and I vaporized under similar conditions from the tungsten coil. The absolute limits of detection (LOD) of the three halogens were in picogram to subnanogram range. A tungsten boat fitted to an ICP-MS spectrometer was used by Okamoto et al. [26] for direct determination of Br in polymers. Samples were previously treated into a cuvette with a solution of alcoholic potassium hydroxide at 180 °C in order to convert all Br species into potassium bromide that is more thermally stable. The vaporization temperature was up to 1600 °C. Maninnen et al. [16] used ETV–ICP-MS for determination of extractable organic Cl. According to the authors, the use of ETV is a good alternative for Cl determination by ICP-MS once this technique is not suitable for the determination of low concentrations of this element by using pneumatic nebulization because of the relatively high ionization potential of this element and polyatomic interferences; the use of ETV would improve the LOD of Cl. Taking into account the potential of ETV for direct analysis of samples that are difficult to decompose and the difficulties related to Cl determination in petroleum coke, the main purpose of this work was the development of an electrothermal vaporization system for direct determination of Cl in petroleum coke by ICP-MS. The second purpose was to develop a method using the ETV to achieve the best performance for Cl determination in petroleum coke. The main parameters evaluated were pyrolysis and vaporization temperatures, Ar flow rate and amount of sample used for analysis. 2. Materials and methods 2.1. Vaporization system A 300 W power halogen lamp (Osram, München, Germany), manufactured using bulb pinch technology [27] was used as heating source. This lamp was employed because its shape could be used as sample holder, where samples can be deposited directly on the bulb. A glass chamber was constructed where the lamp was inserted and fixed. The vaporization system is shown in Fig. 1. The solid sample was introduced in the system through the orifice on the top of the chamber by using a small glass funnel. The orifice is manually closed with a silicon stopper. A glass tube (5 mm i.d.) was used to connect the outlet of the vaporization chamber to the plasma torch in order to transport the vaporized material to the ICP. Solenoid valves (Cole-Parmer, Vernon Hills, USA) (V1 and V2) and a manual valve (MV) were used to control the Ar flow in the system used as purging gas (in the pyrolysis step) or for carrying the vaporized material to the plasma torch (vaporization step) (Fig. 1). The heating
Ar
1
V2 2
2.2. Solutions and samples Solutions were prepared using distilled/deionised water that was purified using a Milli-Q system (18.2 MΩ cm, Millipore, Corp., Bedford, USA). Nitric acid (65% m/m) from Merck (Darmstadt, Germany) was doubly distillated in a quartz still (Millestone, Sorisole, Italy, model duoPUR 2.01E). A 10% (v/v) solution of the purified nitric acid was used for cleaning the vaporization chamber. Certified reference material of coal (SARM 19 from Mintek, South African Reference Materials) and petroleum coke (non-certified) from Centro de Pesquisas e Desenvolvimento Leopoldo Américo Miguez de Mello (CENPES/PETROBRAS) were analyzed. The petroleum coke was dried at 105 °C for 1 h and then ground in agate mortar in order to obtain particles size smaller than 250 μm. Method development and parameters optimization were carried out using petroleum coke. The SARM 19 was used for calibration. The accuracy of the proposed method for Cl determination using ETV–ICP-MS was evaluated by comparing the results with those obtained using other methods [10,6]. 2.3. Procedure For direct analysis of petroleum coke, 0.5 to 7 mg of sample was weighted over a small piece of aluminum foil by the aid of a microspatula. The weighted sample was transferred to the vaporization
Table 1 Operation of valves during the heating cycle of the ETV system used for Cl determination by ICP-MS in petroleum coke.
W V1
temperature of the vaporization chamber was controlled according to the output power that is applied to the lamp. A printed circuit board was developed and used with a commercial interface NI USB-6008 (National Instruments, Austin, USA) to operate this system. Software was developed for temperature and solenoid valves control, allowing the control of the heating program and solenoid valves accordingly. The operation of the valves in each heating step is shown in Table 1. When the heating program is started (pyrolysis step), MV is closed and V1 and V2 are activated. In this step Ar flows through the chamber and volatile components are purged, except the analyte. In the vaporization step, V2 is turned off and MV is switched to the open position and the gaseous Cl is then transported to the ICP-MS instrument. The signal acquisition by the ICP-MS spectrometer starts at the beginning of the vaporization step. The time of pyrolysis and vaporization steps were optimized in view of maximum elimination of sample matrix and complete volatilization of Cl, respectively. The pyrolysis and vaporization temperatures were evaluated in the range 250 to 600 °C and 600 to 1000 °C, respectively. To do so, the ramp and hold time were fixed in 5 s and 20 s, respectively. The software used for controlling the vaporization chamber was developed in LabVIEW environment (National Instruments) for Microsoft Windows. The ETV system was fitted to an ICP-MS spectrometer (PerkinElmer Sciex, model ELAN DRCII, Thornhill, Canada) for Cl determination. The instrumental operation conditions were adjusted for Cl determination in order to achieve the highest analyte signal intensity (blank corrected) and are shown in Table 2. The transient signals were processed in peak area by the WinFAAS 1.0 software.
Heating step
MV ICP-MS
Fig. 1. Electrothermal vaporization system. 1: sample inlet; 2: lamp; V1 and V2: solenoid valves; MV: manual valve; W: waste.
Valves
Function
V1
V2
MV
Pyrolysis
On
On
Off
Vaporization
On
Off
On
Argon flows through the chamber to remove volatile components from sample matrix and to remove traces of air/oxygen Chlorine is volatilized and introduced into the ICP with the aid of argon
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119
A
Table 2 Operational conditions for Cl determination by ETV–ICP-MS. Parameter
Condition
RF power, W Plasma gas flow rate, l min− 1 Auxiliary gas flow rate, l min− 1 Nebulizer gas flow rate, l min− 1 Sampler and skimmer cones Ion monitored Ion lens voltage, V Sweeps/reading Readings/replicate Replicates Data collection mode Dwell time, ms
1350 15 1.20 1.20 Pt 35 + Cl 5.6 1 2000 1 Peak hopping 10
chamber using a small glass funnel inserted in the top of the chamber, allowing placing the sample on the “hole” of the lamp. After closing the chamber, the heating was initialized as well as the valves functioning according to Table 1. Blank signals were measured just by running the complete analysis cycle. Calibration curves were obtained by analyzing increasing amounts of certified reference material (SARM 19).
B c d
b
3. Results and discussion
a
3.1. Optimization and working conditions When the heating program is started, valves V1 and V2 are turned “on” simultaneously while the MV is kept closed (in this step argon does not flow to ICP). In this way, argon flows through the vaporization chamber and the volatile compounds produced in this step (pyrolysis) are removed from the system through V2. At the end of the pyrolysis and beginning of vaporization valve V2 is turned “off” and MV is opened in order to transport the vapor of the analyte to the ICP-MS instrument where the Cl signal is measured. The program for valve control was developed according to the vaporization characteristic of sample matrix and analyte. The holding time of vaporization was set to 20 s. This hold time was established by considering the Cl signal profile. According to Fig. 2B, complete Cl vaporization occurs in less than 20 s and the signal of Cl goes back to baseline. The holding time of pyrolysis was also set to 20 s. Therefore, the total time of heating is 53 s. After the heating cycle is completed, valve V1 is kept “on” up to 100 s, allowing argon to flow through the chamber to aid the cooling of the system. Pyrolysis and vaporization temperatures were established in order to select suitable conditions for direct Cl determination by ETV–ICPMS, avoiding interferences from sample matrix and analyte losses. The temperatures were selected by considering also the Cl signal profile and sensitivity. In order to evaluate the effect of the pyrolysis temperature, the vaporization temperature was set at 900 °C while the pyrolysis temperature was varied from 250 °C to 600 °C in steps of 50 °C. After choosing the pyrolysis temperature, the vaporization temperature was evaluated in the range of 600 to 1000 °C. The pyrolysis and vaporization temperature curves obtained are shown in Fig. 3. The evaluation of the pyrolysis and vaporization temperatures was made by using approximately 2 mg of petroleum coke in each run. The peak areas of Cl signal were normalized to 2 mg. According to Fig. 3, at temperature higher than 350 °C the signal of Cl decreases, indicating losses of Cl. Therefore, the temperature of 350 °C was selected for pyrolysis. With respect to the vaporization temperature, it can be observed in Fig. 3 that the Cl signal remains practically constant even at temperatures higher than 900 °C. Therefore, this temperature was chosen for subsequent Cl measurements. It is important to mention that at 350 °C few volatile compounds are eliminated and most of sample matrix still remains in the vaporization chamber and even after the vaporization step (heating up to 900 °C) the sample matrix is not completely volatilized. Therefore,
e
Fig. 2. Effect of argon flow rate on Cl signal intensity (peak area) (A) and signal profile (B) obtained using the proposed ETV system coupled to ICP-MS. Error bars correspond to analysis of four samples. a, b, c, d and e represent argon flow rates of 1.00, 1.10, 1.20, 1.30 and 1.40 l min− 1, respectively.
between each sample run, the hole on the lamp (sample “holder”) was mechanically cleaned by means of using a tin spatula. After approximately 30 heating cycles, the chamber was opened and residues of samples were completely removed using a flow of argon. At the end of one day of work, the vaporization chamber was cleaned by consecutive heating cycles or by washing with 10% v/v nitric acid. The argon flow used in the vaporization chamber was adapted from the nebulizer gas used in ICP-MS. The argon flow rate was optimized to assure efficient transport of the analyte to the ICP. To do so, the Cl signal intensity (peak area) and the transient signal profile were considered. The pyrolysis and vaporization temperatures were 350 °C and 900 °C, respectively and the petroleum coke mass was 2 mg. The effect of argon flow rate (from 1.00 to 1.40 l min − 1) on Cl
Fig. 3. Pyrolysis and vaporization temperatures for Cl obtained in the presence of petroleum coke.
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signal intensity is shown in Fig. 2A and the effect on signal profile is shown in Fig. 2B. According to Fig. 2A, the signal intensity of Cl decreases with the argon flow rate increasing. However, precision was poor for 1.00 or 1.10 l min− 1 argon flow rates. Additionally, according to the transient signal shown in Fig. 2B, at these conditions the Cl signal takes more time to return to baseline. On the other hand, good peak shape was obtained for argon flow rate of 1.20 or 1.30 l min− 1. Higher intensity of Cl was observed for argon flow rate of 1.20 l min− 1 than for 1.30 l min− 1. At higher argon flow rates the Cl signal intensity decreased probably because plasma conditions were not suitable for ionization process or the analytical zone was dislocated. Therefore, the argon flow rate was fixed in 1.30 l min− 1. 3.2. Effect of sample amount In ICP-MS analysis the sample amount introduced into the plasma can result in severe interferences. On the other hand, as the maximum temperature that could be used in the vaporization system was 1000 °C, too high amount of sample could affect the release of analyte. That is, the sample has to be homogeneously heated in order to vaporize the analyte uniformly. The evaluation of the effect of sample amount was carried out by using 0.5 to 7 mg of petroleum coke and the ETV system submitted to the selected conditions above cited. The results obtained for consecutive analysis of different amounts of sample are shown in Fig. 4. Quite similar signal profiles and good precision were obtained by analyzing aliquots of petroleum coke ranging from 0.5 to 5 mg (Fig. 4) consecutively. For this range of sample good correlation between the signal intensity (peak area) and sample mass was obtained (R was typically 0.99). For higher sample mass, the signal intensity was relatively lower, probably due to inhomogeneous heating of sample and inefficient releasing of Cl or plasma loading. Therefore, the mass of petroleum coke used for analysis should not exceed 5 mg. 3.3. Determination of Cl in petroleum coke In general, analysis of solid materials using ETV has been performed using calibration with aqueous standards [21]. However, depending on the sample, solid reference materials are required for calibration. In the case of matrix matching, the standard solution is delivered onto the residual matrix remaining from the previous sample run [28]. Matrix effect of the solid sample on the analyte signal must be negligible when the calibration is performed by using aqueous standards. However, in the present work this requirement was not fulfilled. Therefore, the calibration was carried out using certified reference material of coal (SARM 19), where the concentration of chlorine (32 μg g − 1) is in the same range of that in petroleum coke, as determined in previous works [6,10]. In order to obtain the calibration curve, increasing amounts of
Table 3 Concentration of Cl in petroleum coke and LOQ obtained by the proposed method and other works. Method
Chlorine, μg g− 1
LOQ, μg g− 1
This worka Pyrohydrolysis and ICP-MS [6] MIC and IC [10]
24.1 ± 2.0 23.6 ± 1.9 22.0 ± 0.8
3.5 3.9 3.8
a Values correspond to mean and standard deviation of four consecutive measurements.
SARM 19 were used. However, the signal profile of Cl in the presence of coal was different of that in the presence of petroleum coke. Results obtained for Cl in petroleum coke using external calibration (with SARM 19) were also not in agreement with those obtained using other methods [6,10]. To overcome this drawback, calibration was then performed using standard additions. In this sense, the mass of petroleum coke was fixed in 1 mg and the amount of certified reference material SARM 19 was increased up to 5 mg. By using this procedure, good correlation between Cl concentration and signal intensity (peak area) was obtained. In this way, the concentration of Cl in petroleum coke could be accurately determined [6,10]. The concentration of Cl found in petroleum coke and the respective limit of quantification (LOQ) estimated as 10s (s is the standard deviation of 10 consecutive measurements of the blank) are shown in Table 3. The Cl concentration found by other methods [6,10] is also shown in this table. In Table 3 it can be observed that the Cl concentration found in the petroleum coke sample by using the developed ETV system is in agreement with the concentrations found by means of other methods [6,10], using pyrohydrolysis and microwave induced combustion (MIC) for sample preparation and ICP-MS and IC for Cl determination. The relative standard deviation (RSD) for Cl was lower than 9% (n = 4) that is satisfactory considering the low mass of the sample (1 mg) used for direct Cl determination by ETV–ICP-MS. The LOQ of the present method is similar to those obtained using other methods for Cl determination [6,10]. However, the use of ETV is advantageous because sample decomposition is not necessary. As a consequence, sample throughput can be drastically improved. By using pyrohydrolysis and MIC for sample decomposition, 4 and 8 samples can be decomposed in 1 h, respectively. Besides, additional time is required for Cl measurement. Alternatively, up to 20 Cl determinations in petroleum coke can be carried out in 1 h by using the proposed method. 4. Conclusions The determination of Cl is of great importance for crude oil industry where fast analytical procedures are required for routine analysis. In this sense, a simple and useful ETV system was developed for direct Cl determination in petroleum coke. The software developed for controlling the temperature/time and solenoid valves allows easy operation of the system. The ETV system enables analysis of up to 20 samples per h, with sufficient sensitivity, precision and accuracy. No reagents are necessary, which is in agreement with green chemistry recommendations. Considering all these aspects, the proposed ETV system is an attractive tool for direct solid analysis using ICP-MS. Acknowledgments The authors are grateful to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for supporting this study. References
Fig. 4. Effect of sample mass on signal intensity (peak area) of Cl.
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Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Development of a vaporization system for direct determination of chlorine in petroleum coke by ICP-MS Fabiane Goldschmidt Antes a, Eduardo Dullius b, Adilson Ben da Costa b, Rolf Fredi Molz b, José Neri Gottfried Paniz a, Erico Marlon Moraes Flores a, Valderi L. Dressler a,⁎ a b
Departamento de Química, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil Programa de Pós-Graduação em Sistemas e Processos Industriais, Universidade de Santa Cruz do Sul, 96815-900, Santa Cruz do Sul, RS, Brazil
a r t i c l e
i n f o
Article history: Received 30 November 2011 Received in revised form 19 March 2012 Accepted 26 March 2012 Available online 30 March 2012 Keywords: Electrothermal vaporization ICP-MS Chlorine Petroleum coke
a b s t r a c t An electrothermal vaporization system (ETV) was developed for direct chlorine (Cl) determination in petroleum coke by inductively coupled plasma mass spectrometry (ICP-MS). The ETV system consists of a high power lamp enclosed in glass chamber equipped with solenoid valves. The system is controlled by a printed circuit board and software developed in LabVIEW environment. The temperature and heating time were optimized. Solenoid valves were used to control the flow of Ar (carrier gas) during pyrolysis and vaporization steps. Pyrolysis and vaporization temperatures were set at 350 °C and 900 °C, respectively; while the Ar flow rate was 1.20 l min − 1 and the plasma power 1300 W. Up to 5 mg of petroleum coke sample can be analyzed. Calibration was carried out by standard addition whereas a certified reference material of coal (SARM 19) was used for that purpose. The relative standard deviation was lower than 9% (n = 4) and the limit of quantification (10s) of Cl was 3.5 μg g− 1. Results obtained for chlorine in petroleum coke sample were in agreement with those obtained using other validated methods for Cl determination. The time of analysis (approximately 2 min per sample), risk of contamination, analyte losses and waste generation are drastically reduced in comparison with techniques that require sample digestion for chlorine determination. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Petroleum coke is a product of petroleum refining process and the chlorine (Cl) content depends on the petroleum origin or source. Petroleum coke is widespread used as fuel, for electrodes manufacturing, production of chemicals, aluminum and steel alloys. The purity of petroleum coke must be evaluated and monitored for its use in these materials [1,2]. Very few works have been published regarding to determination of Cl in petroleum and its refining products, probably because determination of trace amounts of Cl is challenging owing to matrix complexity and Cl losses by volatilization in the sample preparation step. Acid digestion, usually employed for sample decomposition, is not recommended because volatile species of Cl can be produced and then lost. Therefore, most of the published works about Cl determination in petroleum and refining products deal with the employment of combustion methods in closed systems. Decomposition using Schöniger combustion flask [3], combustion bomb [4,5] or microwave induced combustion (MIC) [6,7] has been employed. Pyrohydrolysis was also proposed as an alternative sample decomposition method for further halogens determination [8–10], including Cl. By using
⁎ Corresponding author. Tel./fax: + 55 55 3220 9445. E-mail address: [email protected] (V.L. Dressler). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2012.03.027
combustion or pyrohydrolysis for sample decomposition, Cl determination can be carried out by techniques such as ion chromatography (IC), inductively coupled plasma mass spectrometry (ICP-MS), and inductively coupled plasma optical emission spectrometry (ICP OES). Despite the relatively good results obtained by using the sample preparation methods above cited, they have disadvantages such as high risk of analyte losses, sample contamination, higher cost and relatively low sample throughput. On the other hand, direct solid analysis would be ideal for Cl determination. In fact, direct solid analysis has been employed for Cl determination using inductively coupled plasma mass spectrometry in conjunction with laser ablation (LA) [11–14] and electrothermal vaporization (ETV) [15–18]. These sample introduction accessories have been used for Cl determination in coal by ICP-MS [19–22]. One of the main features of LA-ICP-MS is the possibility to obtain information about spatial distribution of major and trace elements in a sample [23]. However, LA is still not widely used in routine analysis. In the case of ETV, a small amount of solid sample can be placed in the vaporizer, usually a graphite furnace that is heated in order to vaporize the analyte [15]. The resulting sample vapor is transported to the ICP-MS by a carrier gas through a transfer tube. By using ETV as sample introduction system in ICP-MS, the sensitivity is improved and problems related to polyatomic interferences reduced; most part of the water and sample matrix that produced polyatomic ions are eliminated in the ETV before the vaporization of the
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analyte. It was suggested that the use of dry plasma conditions would be better than wet plasma conditions [24]. In fact, interferences caused by polyatomic ions can be strongly minimized using ETV because matrix components are removed during the heating step prior to analyte vaporization. In the case of Cl, this aspect is of great importance because the most abundant isotope of this element ( 35Cl) is strongly affected by interferences caused by 34S1H+ and 16O18O1H+ [10]. Different types of ETV have been produced, including a tungsten coil where Yan et al. [25] determined Cl, Br and I, being the ETV system coupled to an ICP-MS instrument. In this study, sample solutions were deposited on the filament coil, dried, vaporized and then the vapor transported to the plasma. The authors observed that Cl, Br and I vaporized under similar conditions from the tungsten coil. The absolute limits of detection (LOD) of the three halogens were in picogram to subnanogram range. A tungsten boat fitted to an ICP-MS spectrometer was used by Okamoto et al. [26] for direct determination of Br in polymers. Samples were previously treated into a cuvette with a solution of alcoholic potassium hydroxide at 180 °C in order to convert all Br species into potassium bromide that is more thermally stable. The vaporization temperature was up to 1600 °C. Maninnen et al. [16] used ETV–ICP-MS for determination of extractable organic Cl. According to the authors, the use of ETV is a good alternative for Cl determination by ICP-MS once this technique is not suitable for the determination of low concentrations of this element by using pneumatic nebulization because of the relatively high ionization potential of this element and polyatomic interferences; the use of ETV would improve the LOD of Cl. Taking into account the potential of ETV for direct analysis of samples that are difficult to decompose and the difficulties related to Cl determination in petroleum coke, the main purpose of this work was the development of an electrothermal vaporization system for direct determination of Cl in petroleum coke by ICP-MS. The second purpose was to develop a method using the ETV to achieve the best performance for Cl determination in petroleum coke. The main parameters evaluated were pyrolysis and vaporization temperatures, Ar flow rate and amount of sample used for analysis. 2. Materials and methods 2.1. Vaporization system A 300 W power halogen lamp (Osram, München, Germany), manufactured using bulb pinch technology [27] was used as heating source. This lamp was employed because its shape could be used as sample holder, where samples can be deposited directly on the bulb. A glass chamber was constructed where the lamp was inserted and fixed. The vaporization system is shown in Fig. 1. The solid sample was introduced in the system through the orifice on the top of the chamber by using a small glass funnel. The orifice is manually closed with a silicon stopper. A glass tube (5 mm i.d.) was used to connect the outlet of the vaporization chamber to the plasma torch in order to transport the vaporized material to the ICP. Solenoid valves (Cole-Parmer, Vernon Hills, USA) (V1 and V2) and a manual valve (MV) were used to control the Ar flow in the system used as purging gas (in the pyrolysis step) or for carrying the vaporized material to the plasma torch (vaporization step) (Fig. 1). The heating
Ar
1
V2 2
2.2. Solutions and samples Solutions were prepared using distilled/deionised water that was purified using a Milli-Q system (18.2 MΩ cm, Millipore, Corp., Bedford, USA). Nitric acid (65% m/m) from Merck (Darmstadt, Germany) was doubly distillated in a quartz still (Millestone, Sorisole, Italy, model duoPUR 2.01E). A 10% (v/v) solution of the purified nitric acid was used for cleaning the vaporization chamber. Certified reference material of coal (SARM 19 from Mintek, South African Reference Materials) and petroleum coke (non-certified) from Centro de Pesquisas e Desenvolvimento Leopoldo Américo Miguez de Mello (CENPES/PETROBRAS) were analyzed. The petroleum coke was dried at 105 °C for 1 h and then ground in agate mortar in order to obtain particles size smaller than 250 μm. Method development and parameters optimization were carried out using petroleum coke. The SARM 19 was used for calibration. The accuracy of the proposed method for Cl determination using ETV–ICP-MS was evaluated by comparing the results with those obtained using other methods [10,6]. 2.3. Procedure For direct analysis of petroleum coke, 0.5 to 7 mg of sample was weighted over a small piece of aluminum foil by the aid of a microspatula. The weighted sample was transferred to the vaporization
Table 1 Operation of valves during the heating cycle of the ETV system used for Cl determination by ICP-MS in petroleum coke.
W V1
temperature of the vaporization chamber was controlled according to the output power that is applied to the lamp. A printed circuit board was developed and used with a commercial interface NI USB-6008 (National Instruments, Austin, USA) to operate this system. Software was developed for temperature and solenoid valves control, allowing the control of the heating program and solenoid valves accordingly. The operation of the valves in each heating step is shown in Table 1. When the heating program is started (pyrolysis step), MV is closed and V1 and V2 are activated. In this step Ar flows through the chamber and volatile components are purged, except the analyte. In the vaporization step, V2 is turned off and MV is switched to the open position and the gaseous Cl is then transported to the ICP-MS instrument. The signal acquisition by the ICP-MS spectrometer starts at the beginning of the vaporization step. The time of pyrolysis and vaporization steps were optimized in view of maximum elimination of sample matrix and complete volatilization of Cl, respectively. The pyrolysis and vaporization temperatures were evaluated in the range 250 to 600 °C and 600 to 1000 °C, respectively. To do so, the ramp and hold time were fixed in 5 s and 20 s, respectively. The software used for controlling the vaporization chamber was developed in LabVIEW environment (National Instruments) for Microsoft Windows. The ETV system was fitted to an ICP-MS spectrometer (PerkinElmer Sciex, model ELAN DRCII, Thornhill, Canada) for Cl determination. The instrumental operation conditions were adjusted for Cl determination in order to achieve the highest analyte signal intensity (blank corrected) and are shown in Table 2. The transient signals were processed in peak area by the WinFAAS 1.0 software.
Heating step
MV ICP-MS
Fig. 1. Electrothermal vaporization system. 1: sample inlet; 2: lamp; V1 and V2: solenoid valves; MV: manual valve; W: waste.
Valves
Function
V1
V2
MV
Pyrolysis
On
On
Off
Vaporization
On
Off
On
Argon flows through the chamber to remove volatile components from sample matrix and to remove traces of air/oxygen Chlorine is volatilized and introduced into the ICP with the aid of argon
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A
Table 2 Operational conditions for Cl determination by ETV–ICP-MS. Parameter
Condition
RF power, W Plasma gas flow rate, l min− 1 Auxiliary gas flow rate, l min− 1 Nebulizer gas flow rate, l min− 1 Sampler and skimmer cones Ion monitored Ion lens voltage, V Sweeps/reading Readings/replicate Replicates Data collection mode Dwell time, ms
1350 15 1.20 1.20 Pt 35 + Cl 5.6 1 2000 1 Peak hopping 10
chamber using a small glass funnel inserted in the top of the chamber, allowing placing the sample on the “hole” of the lamp. After closing the chamber, the heating was initialized as well as the valves functioning according to Table 1. Blank signals were measured just by running the complete analysis cycle. Calibration curves were obtained by analyzing increasing amounts of certified reference material (SARM 19).
B c d
b
3. Results and discussion
a
3.1. Optimization and working conditions When the heating program is started, valves V1 and V2 are turned “on” simultaneously while the MV is kept closed (in this step argon does not flow to ICP). In this way, argon flows through the vaporization chamber and the volatile compounds produced in this step (pyrolysis) are removed from the system through V2. At the end of the pyrolysis and beginning of vaporization valve V2 is turned “off” and MV is opened in order to transport the vapor of the analyte to the ICP-MS instrument where the Cl signal is measured. The program for valve control was developed according to the vaporization characteristic of sample matrix and analyte. The holding time of vaporization was set to 20 s. This hold time was established by considering the Cl signal profile. According to Fig. 2B, complete Cl vaporization occurs in less than 20 s and the signal of Cl goes back to baseline. The holding time of pyrolysis was also set to 20 s. Therefore, the total time of heating is 53 s. After the heating cycle is completed, valve V1 is kept “on” up to 100 s, allowing argon to flow through the chamber to aid the cooling of the system. Pyrolysis and vaporization temperatures were established in order to select suitable conditions for direct Cl determination by ETV–ICPMS, avoiding interferences from sample matrix and analyte losses. The temperatures were selected by considering also the Cl signal profile and sensitivity. In order to evaluate the effect of the pyrolysis temperature, the vaporization temperature was set at 900 °C while the pyrolysis temperature was varied from 250 °C to 600 °C in steps of 50 °C. After choosing the pyrolysis temperature, the vaporization temperature was evaluated in the range of 600 to 1000 °C. The pyrolysis and vaporization temperature curves obtained are shown in Fig. 3. The evaluation of the pyrolysis and vaporization temperatures was made by using approximately 2 mg of petroleum coke in each run. The peak areas of Cl signal were normalized to 2 mg. According to Fig. 3, at temperature higher than 350 °C the signal of Cl decreases, indicating losses of Cl. Therefore, the temperature of 350 °C was selected for pyrolysis. With respect to the vaporization temperature, it can be observed in Fig. 3 that the Cl signal remains practically constant even at temperatures higher than 900 °C. Therefore, this temperature was chosen for subsequent Cl measurements. It is important to mention that at 350 °C few volatile compounds are eliminated and most of sample matrix still remains in the vaporization chamber and even after the vaporization step (heating up to 900 °C) the sample matrix is not completely volatilized. Therefore,
e
Fig. 2. Effect of argon flow rate on Cl signal intensity (peak area) (A) and signal profile (B) obtained using the proposed ETV system coupled to ICP-MS. Error bars correspond to analysis of four samples. a, b, c, d and e represent argon flow rates of 1.00, 1.10, 1.20, 1.30 and 1.40 l min− 1, respectively.
between each sample run, the hole on the lamp (sample “holder”) was mechanically cleaned by means of using a tin spatula. After approximately 30 heating cycles, the chamber was opened and residues of samples were completely removed using a flow of argon. At the end of one day of work, the vaporization chamber was cleaned by consecutive heating cycles or by washing with 10% v/v nitric acid. The argon flow used in the vaporization chamber was adapted from the nebulizer gas used in ICP-MS. The argon flow rate was optimized to assure efficient transport of the analyte to the ICP. To do so, the Cl signal intensity (peak area) and the transient signal profile were considered. The pyrolysis and vaporization temperatures were 350 °C and 900 °C, respectively and the petroleum coke mass was 2 mg. The effect of argon flow rate (from 1.00 to 1.40 l min − 1) on Cl
Fig. 3. Pyrolysis and vaporization temperatures for Cl obtained in the presence of petroleum coke.
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signal intensity is shown in Fig. 2A and the effect on signal profile is shown in Fig. 2B. According to Fig. 2A, the signal intensity of Cl decreases with the argon flow rate increasing. However, precision was poor for 1.00 or 1.10 l min− 1 argon flow rates. Additionally, according to the transient signal shown in Fig. 2B, at these conditions the Cl signal takes more time to return to baseline. On the other hand, good peak shape was obtained for argon flow rate of 1.20 or 1.30 l min− 1. Higher intensity of Cl was observed for argon flow rate of 1.20 l min− 1 than for 1.30 l min− 1. At higher argon flow rates the Cl signal intensity decreased probably because plasma conditions were not suitable for ionization process or the analytical zone was dislocated. Therefore, the argon flow rate was fixed in 1.30 l min− 1. 3.2. Effect of sample amount In ICP-MS analysis the sample amount introduced into the plasma can result in severe interferences. On the other hand, as the maximum temperature that could be used in the vaporization system was 1000 °C, too high amount of sample could affect the release of analyte. That is, the sample has to be homogeneously heated in order to vaporize the analyte uniformly. The evaluation of the effect of sample amount was carried out by using 0.5 to 7 mg of petroleum coke and the ETV system submitted to the selected conditions above cited. The results obtained for consecutive analysis of different amounts of sample are shown in Fig. 4. Quite similar signal profiles and good precision were obtained by analyzing aliquots of petroleum coke ranging from 0.5 to 5 mg (Fig. 4) consecutively. For this range of sample good correlation between the signal intensity (peak area) and sample mass was obtained (R was typically 0.99). For higher sample mass, the signal intensity was relatively lower, probably due to inhomogeneous heating of sample and inefficient releasing of Cl or plasma loading. Therefore, the mass of petroleum coke used for analysis should not exceed 5 mg. 3.3. Determination of Cl in petroleum coke In general, analysis of solid materials using ETV has been performed using calibration with aqueous standards [21]. However, depending on the sample, solid reference materials are required for calibration. In the case of matrix matching, the standard solution is delivered onto the residual matrix remaining from the previous sample run [28]. Matrix effect of the solid sample on the analyte signal must be negligible when the calibration is performed by using aqueous standards. However, in the present work this requirement was not fulfilled. Therefore, the calibration was carried out using certified reference material of coal (SARM 19), where the concentration of chlorine (32 μg g − 1) is in the same range of that in petroleum coke, as determined in previous works [6,10]. In order to obtain the calibration curve, increasing amounts of
Table 3 Concentration of Cl in petroleum coke and LOQ obtained by the proposed method and other works. Method
Chlorine, μg g− 1
LOQ, μg g− 1
This worka Pyrohydrolysis and ICP-MS [6] MIC and IC [10]
24.1 ± 2.0 23.6 ± 1.9 22.0 ± 0.8
3.5 3.9 3.8
a Values correspond to mean and standard deviation of four consecutive measurements.
SARM 19 were used. However, the signal profile of Cl in the presence of coal was different of that in the presence of petroleum coke. Results obtained for Cl in petroleum coke using external calibration (with SARM 19) were also not in agreement with those obtained using other methods [6,10]. To overcome this drawback, calibration was then performed using standard additions. In this sense, the mass of petroleum coke was fixed in 1 mg and the amount of certified reference material SARM 19 was increased up to 5 mg. By using this procedure, good correlation between Cl concentration and signal intensity (peak area) was obtained. In this way, the concentration of Cl in petroleum coke could be accurately determined [6,10]. The concentration of Cl found in petroleum coke and the respective limit of quantification (LOQ) estimated as 10s (s is the standard deviation of 10 consecutive measurements of the blank) are shown in Table 3. The Cl concentration found by other methods [6,10] is also shown in this table. In Table 3 it can be observed that the Cl concentration found in the petroleum coke sample by using the developed ETV system is in agreement with the concentrations found by means of other methods [6,10], using pyrohydrolysis and microwave induced combustion (MIC) for sample preparation and ICP-MS and IC for Cl determination. The relative standard deviation (RSD) for Cl was lower than 9% (n = 4) that is satisfactory considering the low mass of the sample (1 mg) used for direct Cl determination by ETV–ICP-MS. The LOQ of the present method is similar to those obtained using other methods for Cl determination [6,10]. However, the use of ETV is advantageous because sample decomposition is not necessary. As a consequence, sample throughput can be drastically improved. By using pyrohydrolysis and MIC for sample decomposition, 4 and 8 samples can be decomposed in 1 h, respectively. Besides, additional time is required for Cl measurement. Alternatively, up to 20 Cl determinations in petroleum coke can be carried out in 1 h by using the proposed method. 4. Conclusions The determination of Cl is of great importance for crude oil industry where fast analytical procedures are required for routine analysis. In this sense, a simple and useful ETV system was developed for direct Cl determination in petroleum coke. The software developed for controlling the temperature/time and solenoid valves allows easy operation of the system. The ETV system enables analysis of up to 20 samples per h, with sufficient sensitivity, precision and accuracy. No reagents are necessary, which is in agreement with green chemistry recommendations. Considering all these aspects, the proposed ETV system is an attractive tool for direct solid analysis using ICP-MS. Acknowledgments The authors are grateful to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for supporting this study. References
Fig. 4. Effect of sample mass on signal intensity (peak area) of Cl.
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